BMR for Women Age 65+ at 50 kg — Mifflin-St Jeor

Basal metabolic rate (BMR) is the energy your body uses at complete rest — to keep your heart beating, your lungs breathing, your body warm and your cells maintained — before any movement, digestion or exercise is counted. For a 65 and over-year-old woman weighing 50 kg (110 lb), the Mifflin-St Jeor equation gives a BMR of 1,030 kcal/day, worked step by step below using a height of 165 cm. That figure is an estimate built from a published formula, not a personal prescription.

BMR is the foundation of every calorie target. Total daily energy expenditure (TDEE) is BMR multiplied by an activity factor, and cutting, maintenance and bulking targets are all set relative to TDEE. Getting BMR right therefore matters more than it looks: a 200 kcal error in BMR becomes a 200–380 kcal error in TDEE once the activity multiplier is applied. This page shows the calculation in full so you can check it, and shows how the same person's BMR changes with weight and age, and how it converts into daily calorie needs at each activity level.

At 65 and over, resting metabolism is lower than at any earlier age — the Mifflin-St Jeor age term puts BMR about 235 kcal a day below its age-21 value at the same weight and height — but the number that matters most at this age is not the calorie figure, it is muscle. Muscle mass, strength and balance determine independence, fall risk and recovery from illness, and all three respond to resistance training and adequate protein even in the seventies and eighties. The BMR below is a useful anchor for a calorie target, but it should never be used to justify eating very little: under-nutrition is a bigger risk than over-nutrition for many older adults.

Calculate your own BMR

Your weight, height and age are almost certainly different from the example here, so run your own numbers with the BMR calculator. It uses the same Mifflin-St Jeor equation shown on this page. To turn BMR into a daily calorie target, use the TDEE calculator, which applies the activity multipliers.

Related pages and tools:

The formula: Mifflin-St Jeor (1990)

The Mifflin-St Jeor equation is the most widely validated formula for estimating resting energy expenditure in adults. It was published by Mifflin et al. in the American Journal of Clinical Nutrition in 1990 (Mifflin MD et al., Am J Clin Nutr 1990;51(2):241–247), and it is the equation most calculators and many clinical settings use as a starting point. For women it is:

BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161

For men the final term is +5 instead of − 161. Working through the example for a 65 and over-year-old woman at 50 kg and 165 cm:

  • 10 × 50 kg = 500 kcal
  • 6.25 × 165 cm = 1,031 kcal
  • 5 × 68 years = 340 kcal
  • Constant: − 161 kcal

So BMR = 500 + 1,031 − 340 − 161 = 1,030 kcal/day. Every term has a meaning: weight contributes the most because a larger body costs more energy to maintain; height adds a smaller amount; age subtracts, because resting metabolism declines slowly with age; and the sex constant reflects average differences in body composition between men and women at the same weight and height.

How BMR changes with weight

At the same age (68 years) and height (165 cm), BMR rises by exactly 10 kcal a day for every extra kilogram of body weight, because the weight term is 10 × weight:

Body weight BMR
50 kg
60 kg 1,130.25 kcal/day
70 kg 1,230.25 kcal/day
80 kg 1,330.25 kcal/day
90 kg 1,430.25 kcal/day
100 kg 1,530.25 kcal/day

That 10-kcal-per-kilogram slope is also why BMR falls as you lose weight. A woman who drops 10 kg lowers BMR by about 100 kcal a day, and TDEE falls by 120–190 kcal a day depending on activity level. Calorie targets therefore need recalculating every 5 kg or so of weight change — a target set at your starting weight quietly becomes a smaller deficit, or even a surplus, as the weight comes off.

At 50 kg (110 lb) this is the lightest weight in the BMR tables, and the numbers are tight: a sedentary TDEE of 1,236 kcal a day leaves almost no room for a deficit, because a 500 kcal cut would land at 1,200 kcal — at or below the 1,200 kcal safety floor. If fat loss is the goal at this weight, the honest answer is usually a smaller deficit, more activity, or both, rather than a lower intake. The BMI at this height and weight is 18.4, in the underweight range, so a clinician's input is worth having before any weight-loss plan at this weight.

How BMR changes with age

Holding weight (50 kg) and height (165 cm) fixed, the age term alone lowers BMR by 5 kcal a day for every year. The table below shows the same woman's BMR at different ages:

Age BMR
21 years
32 years 1,210.25 kcal/day
42 years 1,160.25 kcal/day
52 years 1,110.25 kcal/day
62 years 1,060.25 kcal/day
68 years 1,030.25 kcal/day

The gap between 21 and 68 is 235.0 kcal a day — real, but smaller than most people expect. What makes the difference larger in practice is that muscle mass and daily activity usually fall at the same time as age rises, and the equation cannot see either of those. That is why two people of the same sex, age, height and weight can have quite different resting metabolisms, and why the training and activity advice on this page matters as much as the number itself.

From BMR to daily calories: TDEE

BMR is what you burn at rest. Daily life adds movement, and TDEE is BMR multiplied by an activity factor — 1.2 for sedentary, 1.375 for lightly active, 1.55 for moderately active, 1.725 for very active and 1.9 for extremely active. For this 65 and over-year-old woman at 50 kg:

| Activity level | Multiplier | TDEE | |---|---| | | Sedentary | 1.2 | 1,236 kcal/day | | Lightly active | 1.375 | 1,417 kcal/day | | Moderately active | 1.55 | 1,597 kcal/day | | Very active | 1.725 | 1,777 kcal/day | | Extremely active | 1.9 | 1,957 kcal/day | |

A cutting target is usually TDEE minus about 500 kcal a day, and a bulking target is usually TDEE plus about 300 kcal a day — but never below the safety floor of 1,200 kcal a day (women) without medical supervision. For this person, a sedentary cutting target would be 1,200 kcal a day (the safety floor binds here, because TDEE minus 500 would be below it), a moderately active maintenance target would be 1,597 kcal a day, a moderately active cutting target would be 1,200 kcal a day, and a moderately active bulking target would be 1,897 kcal a day.

How to use the 1,030 kcal figure

Use BMR as the anchor, not the target. Nobody should eat at BMR for long: it is the cost of lying still all day, and daily life always adds to it. The practical use of BMR is to build a TDEE estimate honestly — pick the activity level that matches a typical week, not your best week — and then set a goal target from TDEE. If you sit most of the day and train three times a week, you are closer to lightly active than moderately active, and the higher multiplier inflates the target by well over 100 kcal a day.

Use this figure to check that you are eating enough, not just to set a limit. Anchor on BMR, add activity honestly, and if fat loss is the goal, keep the deficit small — about 250–500 kcal a day — and never go below 1,200 kcal a day for women or 1,500 kcal a day for men without medical supervision. Pair any deficit with two or three weekly strength sessions and protein at every meal, and recalculate when weight changes by about 5 kg. If appetite is poor or weight is falling without trying, that is a reason to see a clinician, not to celebrate the loss.

What BMR does not tell you

A BMR estimate is a population average, not a measurement. Individual resting metabolic rate varies by roughly ±10% around the Mifflin-St Jeor prediction because of differences in muscle mass, genetics, thyroid function and health — for this person that means a plausible range of roughly 927–1,133 kcal a day. The equation uses total body weight, so it is less accurate for very muscular people (it underestimates, because muscle burns more at rest than fat) and for people with a high body-fat percentage (it overestimates). The only way to measure BMR directly is indirect calorimetry in a laboratory, which is rarely available outside research or clinical settings.

The estimate carries an error band of roughly ±10% and is at its least precise in older adults, because body composition varies enormously: a lifelong athlete and a lifelong sedentary person can share the same sex, age, height and weight and have BMRs differing by well over 10%. The equation cannot see muscle mass, thyroid function, medication, medical conditions or frailty. It also cannot tell you whether a given weight is healthy for you — that depends on muscle, bone, strength and medical history. Treat the number as a guide and discuss any weight plan with a healthcare professional.

The equation also cannot tell you anything about body composition, fitness or health on its own. A high BMR can belong to a muscular athlete or to a larger person with a high body-fat percentage; a low BMR can belong to a small, healthy person or to someone whose metabolism has adapted downward after a long period of very low intake. Pair BMR with other measures — waist circumference, body-fat estimates, strength, blood markers — before drawing conclusions about health. And never use BMR as an eating target: eating at or below BMR for an extended period without medical supervision risks nutrient deficiency, muscle loss and hormonal disruption.

Frequently asked questions

What is the BMR of a 65 and over-year-old woman weighing 50 kg?

Using the Mifflin-St Jeor equation with a height of 165 cm and age 68 years, BMR is 1,030 kcal/day: 10 × 50 + 6.25 × 165 − 5 × 68 − 161 = 1,030. Your own BMR will differ if your height or exact age differ; use the BMR calculator with your own measurements.

Is Mifflin-St Jeor the most accurate BMR formula?

For most adults it is the best-validated predictive equation available, which is why it is the standard choice for calculators and clinical use. It still carries an error of roughly ±10% for an individual, because it cannot see your muscle mass — for this person that means a plausible range of about 927–1,133 kcal a day. Formulas that use body-fat percentage, such as Katch-McArdle, can do better for very muscular or very lean people — but only if the body-fat figure fed into them is accurate.

Is losing weight safe at 65-plus?

It can be, at a slow pace — about 0.25–0.5 kg a week — with strength training and enough protein to protect muscle, and with a clinician's input if there are medical conditions or medications involved. What is risky is fast weight loss or a very low calorie intake, which can cost muscle and bone. Unintentional weight loss at this age should always be checked by a clinician rather than welcomed.

Should I eat at my BMR to lose weight?

No. BMR is the energy cost of lying still all day; eating at BMR while living a normal active life creates a deficit equal to everything you do beyond lying still, which is usually far too large and unsustainable. Set the target from TDEE instead — for this person, a moderately active cutting target is about 1,200 kcal a day — and never go below 1,200 kcal a day (women) without medical supervision.

How often should I recalculate my BMR?

Recalculate whenever your weight changes by about 5 kg, when your activity level changes for more than a few weeks, or at least once a year as you age. A target based on an old weight gradually stops working: as weight falls, BMR and TDEE fall with it — 10 kcal of BMR per kilogram — so a deficit set months ago shrinks over time. At 50 kg, each 5 kg of weight change moves BMR by 50 kcal a day.

Why is my appetite lower than it used to be?

Appetite commonly falls with age, and so does thirst — both are normal ageing changes, but they make under-eating and dehydration more likely. If you are eating much less than the maintenance figures in the TDEE table and losing weight without trying, see a clinician. Regular meals, protein at each one, and some strength training all help appetite and muscle maintenance.

Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.